The use of electrical measurements for gathering of downhole information, such as logging while drilling (“LWD”), measurement while drilling (“MWD”), and wireline logging system, is well known in the oil industry. Such technology has been utilized to obtain downhole information, such as formation resistivity (or conductivity; the terms “resistivity” and “conductivity”, though reciprocal, are often used interchangeably in the art.), dielectric constant, etc, to determine the petrophysical properties of a subterranean formation and the fluids therein accordingly. The collected downhole information can help delineate hydrocarbon (such as crude oil or gas) and other contents in the porous formation and identify bed boundary in between different formations. It is preferable to keep the borehole in the pay zone (the formation with hydrocarbons) as much as possible so as to maximize the recovery.
Various measurement tools exist for use in LWD, MWD, or wireline logging system. One such tool is a resistivity tool. FIG. 1 shows a conventional resistivity tool, which includes a drill string 100, a pair of transmitters T1 and T2 for transmitting electromagnetic signals through surrounding formation, a pair of receivers R1 and R2 for receiving electromagnetic signals from the pair of transmitters T1 and T2, and a drill bit 112 at a distal end of the drill string 100. Compared to the transmitter T2, the transmitter T1 is closer to the pair of receivers R1 and R2.
In FIG. 1, the drill string 100 rotates and moves in a first bed formation 102 toward a bed boundary 106 between the first bed formation 102 and a second bed formation 104. When the drill string 100 is approaching the bed boundary 106, electromagnetic signals sourced from the transmitter T2 108 start penetrating the bed boundary 106, propagate through the second bed formation 104, and then are received by the pair of receivers R1 and R2. However, at the same time, electromagnetic signals sourced from the transmitter T1 110 still propagate through the first formation 102 only and then are received by the pair of receivers R1 and R2 at the end. Accordingly, the resistivity data measured by the transmitter T1 would be different from the resistivity data measured by the transmitter T2. The difference of measured resistivity indicates the presence of the bed boundary 106.
To keep a resistivity tool staying within the pay zone, a decision of steering the resistivity tool requires not only information of the location of a formation boundary, but also information of its orientation. The resistivity tool shown in FIG. 1 can not identify the orientation of the bed boundary 106 with respect to the drill string 100. Therefore, a “directional” resistivity tool was invented accordingly. The directional resistivity tool can be azimuthally sensitive by collecting logging information on azimuthally different angles while rotating. As shown in FIG. 2, a borehole can be divided into a number of bins (or sectors) 200˜230. Conventionally, the number of bins is 16 or 32. The location and orientation of a formation boundary can be computed based on the correlation results of measured resistivities from each bin. However, mechanical rotation of the directional resistivity tool brings some disadvantages. For example, the vibration and shaking of the directional resistivity tool would significantly affect the accuracy of data measurements. Furthermore, the speed of mechanical rotation has physical limitation.
So far some procedures have been implemented to avoid vibration problems, such as stopping the tool in each bin while conducting measurements in this bin. It may solve the vibration problems, but greatly prolongs the process of data measurements.
As described above, a need exists for an improved apparatus and method for detecting the location and orientation of a bed boundary without requiring the apparatus to rotate mechanically.
A further need exists for an improved apparatus and method for detecting the location and orientation of bed boundary without vibration or shaking issues causing inaccuracy of data measurements.
A further need exists for an improved apparatus and method for detecting the location and orientation of a bed boundary in an efficient way.
The present embodiments of the apparatus and the method meet these needs and improve on the technology.